Keywords
echocardiography - aorta - dissection - bicuspid aortic valve
Introduction
Aortic disease can present as a multitude of conditions, ranging from asymptomatic
aneurysms to life-threatening acute aortic syndromes. While a significant cause of
morbidity and mortality, the incidence of aortic disease is likely underestimated
due to some processes being clinically silent. Thoracic aortic aneurysms (TAA) are
estimated to have an annual incidence of 5 to 10 cases/100,000 patient years, with
more acute symptomatic syndromes like thoracic aortic dissection having an incidence
of 3 to 4 cases/100,000 patient years.[1]
[2] In acute aortic syndromes, and especially in asymptomatic diseases where there may
not be any physical examination cues, echocardiography plays an important role in
diagnosis and serial evaluation. Standard transthoracic echocardiography (TTE) obtains
certain views of the aorta and is useful not only in diagnosis but also in follow-up
screening. Transesophageal echo (TEE) also plays a vital role in obtaining a more
comprehensive view of the aorta, especially when transthoracic views are limited.
In this review, we will highlight echocardiographic evaluation of the normal aorta
and selected aortic pathologies, including tips to improve imaging quality and pitfalls
to avoid that may impede accurate assessment.
Echocardiographic Evaluation of the Normal Aorta
The aorta is divided into five anatomic segments: (1) aortic root, (2) tubular ascending
aorta, (3) aortic arch, (4) descending aorta, and (5) abdominal aorta. The aortic
root is further subdivided into the sinuses of Valsalva and the sinotubular junction,
which is the intersection between the root and the tubular ascending aorta. The aortic
annulus is the most proximal portion of the aortic root, where the leaflets of the
aortic valve are inserted. Just distal to that are the sinuses of Valsalva from which
the coronary arteries originate. The aorta as a whole serves as a dynamic elastic
conduit connecting the cardiac pump to the rest of the organs in the body and thereby
delivers oxygenated blood. Evaluation of the aorta is part of the standardized protocol
when performing a TTE, though it is important to note that not every aforementioned
segment may be adequately visualized with this modality.
The canonical views in which the aorta is visualized with TTE are the parasternal
long axis (PLAX), parasternal short axis, apical view, subcostal view, and suprasternal
view, typically in that order.[3] The PLAX is viewed by placing the patient in the left lateral decubitus position
and the ultrasound probe at the third or fourth intercostal space, adjacent to the
sternum on the left side, with the transducer marker pointing to the right shoulder.
In this window, the parts of the aorta visualized in longitudinal cross-section are
the aortic annulus, sinuses of Valsalva, sinotubular junction, and proximal part of
the tubular ascending aorta ([Fig. 1A]). The descending aorta can also be visualized just below the left atrium, though
in transverse cross-section. A tip to better visualize the tubular ascending aorta
is remaining adjacent to the sternum but going up a rib space. This window allows
for better visualization and is used for measurement of the proximal ascending aorta
([Fig. 1B]). If the probe is rotated 90 degrees with the marker facing the left shoulder, the
left ventricle appears in transverse cross section (short axis), and if the probe
tail is tipped down, the aortic annulus appears in transverse cross-section with visualization
of the aortic valve ([Fig. 1C]). This is useful in helping evaluate the number of aortic valve leaflets. In the
apical five and three chamber view, the aortic annulus and sinuses of Valsalva can
be visualized ([Fig. 1D]), though size measurements are not typically made in this view as the PLAX is preferred
given closer proximity to the probe. This window is useful in obtaining functional
information about the valve, such as stenosis or regurgitation, given better alignment
with the spectral Doppler interrogation. The subcostal view is helpful in evaluating
the descending aorta, which can be visualized by tipping the probe tail up after seeing
the inferior vena cava with the marker pointing in the 3 o'clock position ([Fig. 1E]). Finally, in the suprasternal view, the aortic arch can be visualized, including
the three main branches of the arch if image quality is excellent ([Fig. 1F]).
Fig. 1 Canonical views on transthoracic echo (TTE): (A) parasternal long axis with anatomic and TTE images, (B) parasternal long axis TTE image up a rib space showing ascending aorta, (C) parasternal short axis with anatomic and TTE images, (D) apical five chambers with anatomic and TTE images, (E) subcostal TTE image, (F) suprasternal with anatomic and TTE images. Images (A–D) and (F) adapted from Mitchell et al[3] and image (E) from Evangelista et al.[13] Ab Ao, abdominal aorta; Asc Ao, ascending aorta; AV, aortic valve; CT, computed
tomography; Desc Ao, descending aorta; IAS, interartrial septum; Innom a, innominate
artery; IVS, interventricular septom; LA, left atrium; LAX, long axis; LCCA, left
common carotid artery; LSA, left subclavian artery; LV, left ventricle; LVOT, left
ventricular outflow tract; ME, mid esophageal; MV, mitral valve; PV, pulmonic valve;
RA, right atrium; RV, right ventricle; RVOT, right ventricular outflow tract; SAX,
short axis; TV, tricuspid valve; UE, upper esophageal.
The canonical views in which the aorta is visualized on TEE include mid-esophageal
and upper-esophageal views at different plane angles.[4] At approximately 45 degrees in the mid-esophageal view, the aortic valve can be
seen in transverse cross-section, which is helpful for direct visualization to determine
the number of leaflets ([Fig. 2A]). If the probe is pulled out a few centimeters, at approximately 100 degrees and
with some anteflexion, the proximal ascending aorta can be laid out in a longitudinal
view for diameter measurement ([Fig. 2B]). In the mid-esophageal view at 135 degrees, the aortic valve can be visualized
in long axis, which can be used to measure the left ventricular outflow tract, sinuses
of Valsalva, and sinotubular junction, as well as to assess aortic valve function
([Fig. 2C]). Finally, in the mid-esophageal view, the probe is rotated in clockwise or counterclockwise
fashion until the descending aorta can be viewed, in 0 degrees as short axis and 90
degrees as long axis ([Fig. 2D, E]). As the probe is pulled out, in the upper esophagus, the aortic arch can be visualized
as well ([Fig. 2F]).
Fig. 2 Canonical views on transesophageal echo (TEE): (A) mid-esophageal view at 45 degrees (short axis) with anatomic, three-dimensional
(3D) and TEE images, (B) mid-esophageal view at 100 degrees of the ascending aorta with anatomic, 3D and
TEE images (C) mid-esophageal view at 135 degrees (long axis) with anatomic, 3D and TEE images,
(D) mid-esophageal view at 0 degrees (short axis) of the descending aorta with anatomic,
3D and TEE images, (E) mid-esophageal view at 90 degrees (long axis) of the descending aorta with anatomic,
3D and TEE images, (F) upper esophageal view at 0 degrees of the aortic arch with anatomic, 3D and TEE
images. AV, aortic valve; LAX, long axis, ME, mid esophageal; SAX, short axis; UE,
upper esophageal. Image courtesy: Hahn et al.[4]
When measuring the aorta by echocardiography, it is important to emphasize that normal
aortic size can vary from person to person based on certain patient characteristics.
Roman et al[5] studied the aortic root diameters of 135 normal adults and 52 normal infants and
children and compared several characteristics including age, gender, body habitus,
blood pressure, and stroke volume. Of these characteristics, age and body surface
area (BSA) were found to be the most significant factors in determining normal aortic
size. [Fig. 3] shows normal ranges of the sinuses of Valsalva diameter for three different age
groups: [Fig. 3A] is for ages 1 to 15 years, [Fig. 3B] is for 20 to 39 years, and [Fig. 3C] is for ≥40 years. Based on this chart, for example, a 50-year-old patient with a
BSA of 2.2 would have an upper limit of normal of 4.2 cm, whereas a 30-year-old patient
with a BSA of 1.6 would have an upper limit of normal of 3.2 cm.
Fig. 3 Normal ranges of Sinuses of Valsalva diameter with respect to body surface area across
three age groups (left to right: 1–15 years, 20–39 years, and >40 years). SEE, standard
error of the estimate. Image courtesies: Roman et al[5] and Goldstein et al.[8]
Normal values of the ascending aorta are also based on certain patient characteristics.
Davies et al[6] followed 803 patients with thoracic aortic aneurysms with serial multimodal imaging
and found that aortic diameter indexed to BSA was a more important predictor of outcomes
than absolute diameter. In a more recent study, the same group indexed aortic size
to height in 780 patients and found that height-based ratio was as good or better
at predicting outcomes ([Fig. 4]).[7] Therefore, it is important not to fall into the pitfall of “one size fits all” for
normal aortic size dimensions.
Fig. 4 Risk of complications (aortic dissection, rupture, and death) in patients with ascending
aortic aneurysm as a function of aortic diameter (horizontal axis) and height (vertical
axis), with the aortic height index given within the figure. Image courtesy: Zafar
et al.[7]
It is also important to emphasize the proper technique for measuring the normal size
of the aorta. There are three key techniques that should be followed to ensure accurate
measurement: (1) timing of the cardiac cycle, (2) edges of measurement, and (3) echocardiographic
windows.[8] With respect to timing of the cardiac cycle, all measurements of the aorta except
the aortic annulus are recommended to be done in end diastole. The aorta has elastic
properties that enable it to accommodate the ejection of blood from the left ventricle,
and thus varies slightly in diameter between systole and diastole. Therefore, consistent
measures must be used to compare with normal values for each study, as well as to
compare follow-up studies in a single patient. With respect to edges of measurements,
aortic root and ascending aorta measurements should be done from leading edge to leading
edge. For TTE, this means measurement of the outside of the anterior wall to the inside
of the posterior wall, and for TEE, this would be reversed since the probe is behind
the heart, that is, outside of the posterior wall to the inside of the anterior wall
([Fig. 5A, B]). Leading edge to leading edge was originally recommended by the American Society
of Echocardiography (ASE) in 1978 due to the belief that this technique reduces inaccuracies
in measurement that could arise from blooming artifact due to the aortic wall being
a bright reflector.[9] Large studies that were done after those ASE recommendations and multiple guidelines
have since used the leading edge to leading edge in end-diastole technique, and thus
most available prognostic data have stemmed from this convention. Moreover, Muraru
et al[10] performed aortic measurements using inner edge to inner edge verses leading edge
to leading edge with two-dimensional (2D) echocardiography in 218 healthy volunteers
and found that inner edge measurements produced significantly smaller diameters as
compared with leading edge diameters.[10] The only exception to this convention, however, is the aortic annulus, which is
measured from inner edge to inner edge, where the leaflets insert, in midsystole when
the leaflet tips are open ([Fig. 5C]). Finally, the TTE window that measurements are done is the PLAX, where the aorta
lies perpendicular to the ultrasound probe.
Fig. 5 (A) Parasternal long axis on transthoracic echo (TTE) showing leading edge to leading
edge measurement, from outside the anterior wall to the inside of the posterior wall,
(B) transesophageal echo (TEE) mid-esophageal view at 135 degrees (long axis) showing
leading edge to leading edge measurement, from outside the posterior wall to the inside
of the anterior wall, (C) parasternal long axis on TTE showing inner edge to inner edge measurement of the
(1) LVOT and (2) aortic annulus in midsystole with open leaflets. A, anterior, AV,
aortic valve; IVS, interventricular septum; LA, left atrium; LV, left ventricle; LVOT,
left ventricular outflow tract; MV, mitral valve; P, posterior; RV, right ventricle.
Red lines added for this figure to demonstrate proper measuring technique. Image courtesy:
Mitchell et al.[3]
Echocardiography is a useful imaging modality because it is cost effective, portable,
and widely available. Other than echocardiography, however, computed tomography (CT)
and magnetic resonance imaging (MRI) can also be used as multimodal approaches to
imaging the aorta, though these techniques may not be as cost effective or accessible.
CT angiography (CTA) can provide a rapid, noninvasive tool that allows visualization
of the entire aorta, though it exposes the patient to radiation and contrast and cannot
provide functional information about the aortic valve. MRI can also provide visualization
of the entire aorta without any radiation exposure, though it is costly and less widely
available, and the patient has to remain still in a supine position for a much longer
time. Interestingly, these imaging modalities use an inner edge to inner edge measuring
technique. When comparing echocardiographic measurement convention, there is good
correlation with inner edge to inner edge measurements in CT/MRI. In fact, the three
modalities were compared by Rodríguez-Palomares et al[11] in 2016 in a study of 140 patients with aortic disease. They found that when comparing
leading edge to leading edge technique in TTE to inner edge to inner edge technique
in CT/MRI, there was good accuracy and reproducibility of aortic diameter estimation.
When inner edge to inner edge was used in TTE, however, it underestimated the diameters
as compared with CT/MRI. It is therefore worth highlighting that in addition to TTE,
CT and MRI can be useful tools for physicians to utilize in aortic evaluation, especially
when visualization of the entire aorta is necessary as this cannot be achieved with
TTE alone.
Echocardiographic Evaluation of Selected Aortic Pathologies
Aortic Dissection
Aortic dissection, which falls under the category of acute aortic syndromes, occurs
when there is a tear in the intimal layer of the aorta, and pulsatile blood flow enters
causing a separation of the layers. Subsequently, the tear can propagate, resulting
in two lumens of blood flow, a true and false lumen. Aortic dissections can be categorized
based on the Stanford classification system into Type A and Type B, with Type A defined
as dissections of the ascending aorta ± the arch and descending aorta, and Type B
defined as dissection of the aorta below the level of the left subclavian artery only.[12] Type A dissections are particularly dangerous, as they can propagate into the coronary
arteries causing myocardial infarction, disrupt the aortic valve resulting in significant
valvular regurgitation, or extend into the pericardium resulting in acute pericardial
effusion and cardiac tamponade. While CT has traditionally been the initial diagnostic
test of choice, echocardiography provides valuable information in this condition,
and the two modalities can be used in combination for evaluation and treatment planning.
CT has the advantage that it can allow visualization of the entire aorta and branch
vessels, but it does not provide any functional data that echocardiography can provide
such as the mechanism and severity of aortic regurgitation, presence of wall motion
abnormalities to suggest coronary involvement, or evidence of hemodynamically significant
pericardial effusion. Furthermore, in patients who are hemodynamically too unstable
to go to a CT scanner, or cannot get contrast due to renal disease, TEE provides a
portable diagnostic tool that can be performed at the bedside with a sensitivity of
86 to 100% and a specificity of 90 to 100%.[13] It is important to keep in mind, however, that TEE has a blind spot at the distal
ascending aorta and proximal aortic arch because the air-filled tracheal carina interferes
with ultrasound beam propagation from the esophagus to the heart. [Table 1] describes the different criteria for diagnosis of dissection that can be detected
by echocardiography.
Table 1
Role of echocardiography in detecting evidence of aortic dissection and echocardiographic
definitions of main findings (adapted from Goldstein et al[8])
|
Diagnostic goals
|
Definition by echocardiography
|
|
Identify presence of a dissection flap
|
Flap diving two lumens
|
|
Define extension of aortic dissection
|
Extension of the flap and true/false lumens in the aortic root (ascending/arch/descending
abdominal aorta)
|
|
Identify true lumen
|
Systolic expansion, diastolic collapse, systolic jet directed away from the lumen,
absence of spontaneous contrast, and forward systolic flow
|
|
Identify false lumen
|
Diastolic diameter increase, spontaneous contrast and or thrombus formation, and reverse/delayed
or absent flow
|
|
Identify presence of false luminal thrombosis
|
Mass separated from the intimal flap and aortic wall inside the false lumen
|
|
Localize entry tear
|
Disruption of the flap continuity with fluttering or ruptured intimal borders; color
Doppler shows flow through the tear
|
|
Assess presence, severity and mechanisms of aortic regurgitation
|
Anatomic definition of the valve (bicuspid, degenerated, and normal with/without prolapse
of one cusp); dilation of different segments of the aorta; flap invagination into
the valve; and severity by classic echocardiographic criteria
|
|
Assess coronary artery involvement
|
Flap invaginated into the coronary ostium, flap obstructing the ostium, absence of
coronary flow, and new regional wall motion abnormalities
|
|
Assess side-branch involvement
|
Flap invaginated into the aortic branches
|
|
Detect pericardial and/or pleural effusion
|
Echo-free space in the pericardium/pleura
|
|
Detect signs of cardiac tamponade
|
Classic echocardiographic and Doppler signs of tamponade
|
One of the pitfalls of aortic imaging by echocardiography to note is reverberation
artifact which can create the false appearance of a dissection flap. A reverberation
artifact occurs when there is a strong reflector of ultrasound beams within the scanning
sector, such as the wall of the aorta, which causes the ultrasound beams to bounce
back and forth causing the initial structure to reappear with weaker intensity, farther
away from the probe.[14] This can result in a diagnostic dilemma in which the reader may confuse an artifact
for a dissection flap. There are several tips and tricks that can be used to distinguish
artifact from true dissection. Color Doppler imaging can help with this distinction,
as it will respect the boundaries of a true dissection flap and show two distinct
areas of different color flow in the true lumen and the false lumen ([Video 1A, B]; available in the online version only).[15] If the false lumen is thrombosed, it may show minimal or no color flow. However,
color Doppler flow patterns will traverse the entirety of the aortic lumen without
respecting a “flap” that is really reverberation artifact ([Video 1C, D]; available in the online version only). M-mode interrogation can also be performed
through the aorta to see if there is independent motion of the “flap.” A reverberation
artifact will not demonstrate independent motion and will perfectly mimic movement
of the aortic wall, whereas a true dissection flap should demonstrate independent
fluttering motion as blood flows through it ([Fig. 6]).
Fig. 6 M-mode of the aorta in cross section on transesophageal echo showing reverberation
artifact. The “dissection flap” (red arrow) shows perfect concordance with the aortic
wall (blue arrow), suggesting artifact rather than true dissection. Image courtesy:
Yale Echocardiography Laboratory.
Video 1 (A) Transesophageal echo (TEE) view of the aorta in transverse cross section showing
a dissection flap with a true lumen and a false lumen that is thrombosed, (B) TEE view from [Video 1A] with color Doppler, showing respect of the flap boundaries and no flow in the thrombosed
false lumen, (C) TEE view of the aorta in transverse cross section showing a reverberation artifact
“flap,” (D) TEE view from [Video 1C] with color Doppler, showing color traversing the entirety of the lumen. Videos from
the Yale Echocardiography Laboratory. 2D, two-dimensional; bpm, beats per minute.
Lastly, contrast agents can be used to follow blood flow, and visualization of different
lumens can be identified. Similar to color flow, contrast will either follow or not
follow the boundaries of the flaps in the cases of true dissection versus artifact,
respectively. While TTE has less sensitivity than TEE for dissection, the addition
of contrast can increase TTE sensitivity from 70–85 to 93% assuming sufficient image
quality.[8]
[13]
Bicuspid Aortic Valve
Bicuspid aortic valve (BAV) is a congenital cardiac abnormality in which there are
two aortic valve cusps instead of three. It has a prevalence of 1 to 2%, with the
most common type being fusion of the right and left coronary cusp, accounting for
70 to 80% of BAVs.[16] Diagnosis of this condition is of paramount importance because these valves can
degenerate at a much younger age leading to symptomatic stenosis or regurgitation,
and patients can have associated aortic aneurysms with literature reports of prevalence
varying from 20 to 84%.[16] The primary mode of diagnosis of BAV is echocardiography. The best view to diagnose
BAV is in the parasternal short axis. It is important to highlight that one pitfall
is viewing the valve in diastole as opposed to systole. If it is visualized in diastole
when the valve is closed, it can still appear trileaflet even if it is bicuspid due
to the presence of a raphé or fusion line between two leaflets. Therefore, it should
be viewed in systole when the valve is open, to visualize the commissural fusion with
raphé and “fish mouth” or football-shaped appearance of the valve orifice ([Fig. 7]).[17]
Fig. 7 (A) Aortic valve during diastole showing the appearance of three leaflets, (B) same aortic valve in systole showing true bicuspid valve. Image courtesy: Santarpia
et al.[17]
Another tip to help distinguish bicuspid from trileaflet valve is M-mode interrogation
through the aortic valve in PLAX. Normally, the closure line of the aortic valve in
diastole should be in the center of the aorta, but with bicuspid valves, one of two
valve leaflets is usually bigger, and so the closure line will be eccentric ([Fig. 8A, B]).[18] Furthermore, in the PLAX, doming of the leaflets can be seen ([Video 2A]; available in the online version only). Finally, a tip off suggesting possible bicuspid
valve is eccentric aortic regurgitation ([Video 2B]; available in the online version only).
Video 2 (A) Parasternal long axis transthoracic echo (TTE) view showing doming of the aortic
valve leaflets, (B) parasternal long axis TTE view with color Doppler showing an eccentric jet of aortic
regurgitation. Source Yale Echocardiography Laboratory.
Fig. 8 (A) Central closure suggestive of a tricuspid aortic valve, (B) eccentric closure suggesting bicuspid aortic valve. Image courtesy: Yale Echocardiography
Laboratory.
Conclusion
Echocardiography is a vital imaging modality in the evaluation of the thoracic aorta.
It is important to keep in mind standard techniques for visualization and measurements,
such as leading edge to leading edge, end diastole, and appropriate window, to accurately
diagnose and follow-up patients. Different techniques, such as M-mode, color Doppler
flow, or contrast echocardiography, can be useful tools to help distinguish between
true disease and artifact. Clinicians should be aware of pitfalls in interpreting
echocardiographic studies that may lead to misdiagnosis or delay in diagnosis. CT
and MRI are other multimodal approaches for diagnosis and evaluation of aortic pathology
when visualization on TTE or TEE is suboptimal or further imaging is required.
